Reverse-flow reactors (RFRs) are known in the art, for example, Wulff pyrolysis and regenerative reactor and other regenerative reactors, including regenerative thermal oxidizers (RTO). These reactors are typically used to execute cyclic, batch-generation, high temperature chemistry. Regenerative reactor cycles are either symmetric (same chemistry or reaction in both directions) or asymmetric (chemistry or reaction changes with step in cycle). Symmetric cycles are typically used for relatively mild exothermic chemistry, examples being regenerative thermal oxidation (RTO) and autothermal reforming (ATR). Asymmetric cycles are typically used to execute endothermic chemistry, and the desired endothermic chemistry is paired with a different chemistry that is exothermic (typically combustion) to provide heat of reaction for the endothermic reaction. Examples of asymmetric cycles are Wulff pyrolysis processes and pressure swing reforming processes (PSR).
To operate the RFRs, various operational features should be considered. For instance, one feature of RFRs is a gas hourly space velocity, which is the space velocity of a gas over a given reactor volume. Typically, a high gas hourly space velocity (and hence reactor productivity) has a small reactor cycle time, while low has hourly space velocity has a longer reactor cycle time. For pyrolysis processes using a RFR, high velocities are needed to achieve short residence times that facilitate conversion to preferred products. A second feature is that the volume of gas remaining in the RFR at the end of one cycle (void volume) should be managed, e.g., swept out, before the beginning of the next cycle, which gas-volume management may result in inefficiency and additional costs. A third feature is that bed structures (packing) needed to provide rapid heat transfer (for sharp thermal gradients and resulting high efficiency) also results in high pressure drop. Thus, the RFR design should consider space velocity, void volume, and packing properties to properly manage the system. Accordingly, certain drawbacks in conventional RFRs, such as properties of conventional packing and long cycle times, have prevented these reactors from being broadly used in the energy and petrochemical fields.
RFRs have historically utilized different packing material in the bed structures. Typically, these reverse-flow reactors utilize checker brick, pebble beds or other available packing. This type of bed structure typically has low geometric surface area (aV), which minimizes pressure drop per unit of reactor length, but also reduces volumetric heat transfer rate. One basic principle of an asymmetric reverse flow reactor is that heat is stored in one step and is used to accomplish a desired endothermic chemistry in a second step. Thus, the amount of desired chemistry that can be achieved, per volume of reactor, is directly related to the volumetric heat transfer rate. Lower heat transfer rates thereby require larger reactor volumes to achieve the same amount of desired chemical production. Lower heat transfer rates may inadequately capture heat from RFR streams, leading to greater sensible heat loss and consequently lower efficiency. Lower heat transfer rates may also lead to longer cycle times, as the stored heat is used more slowly, and therefore lasts longer for a given bed temperature specification. Historic RFR's, with low-aV checker-brick or pebble bed packing, are larger (e.g., longer and more capital intensive) and have cycle times of two minutes or greater. As such, these reactors limit reactor efficiency and practical reactor size.
As an enhancement, some RFRs may utilize engineered packing within the bed structure. The engineering packing may include a material provided in a specific configuration, such as a honeycomb, ceramic foams or the like. These engineered packings have a higher geometric surface area (aV), as compared to other bed structures. The use of this type of packing allows for higher gas hourly space velocity, higher volumetric reactor productivity, higher thermal efficiency, and smaller, more economical reactors. However, these more-economical reactors use heat more rapidly and thus may require reduced cycle times. Pressure swing reforming processes (PSR) are an example of such a preferred RFR.
Further, as a result of using this type of packing material, the size of the reactor may be reduced, which provides significant capital cost savings. However, adjusting the packing material of the reactors impacts other operational features. For instance, the increase in volumetric surface area (aV) is typically accomplished using smaller flow channels that result in higher pressure drop per unit of reactor length. To compensate for this, these enhanced RFR's are configured to have short lengths. When applied to large petrochemical applications, diameter is increased to enable high productivity, but length is limited by pressure drop, thus leading to a high ratio for diameter per length (D/L). Conventional reactor designs typically collect fluids emerging from a bed and duct those fluids to some external valve. The volume of such ducting is in some proportion to the reactor diameter, because the ducting needs to collect gas from the entire diameter. Thus, for a conventional reactor having a high D/L ratio, the volume of ducting can be very large compared to the volume inside the bed. Use of a conventional reactor design for an enhanced RFR thus results in large void volumes (primarily in the ducting), which creates problems for gas volume management.
Unfortunately, conventional reactor valve systems have certain limitations that do not operate properly for enhanced, high-productivity reactors (e.g., compact reactors employing short cycle times). For instance, conventional reactor valve systems typically fail to meet the durability requirements of RFRs and may not handle the short cycle times. Petrochemical valves can have maximum cycle lifetimes on the order of 500,000 cycles, which correspond to less than one year of operation—inadequate for petrochemical use involving rapid cycle times. In addition, conventional valves are placed outside the reactor and use manifolding to carry gases between the bed and the valve, while providing uniform flow distribution across the bed. Given the wide and short beds of RFRs, this manifolding holds a large gas volume that has to be managed on every cycle change.
As an example, Japanese Patent Application No. 280,921/1999 to Taga discloses a high-temperature heat exchanger with a plurality of high-temperature gas switching poppet valve pairs which control the flow of high-temperature preheated air and high-temperature waste gas over a regenerative heat exchange element.
U.S. Patent Application Publication No. 2009/0008292 to Keusenkothen et al. discloses pyrolyzing hydrocarbons in a reverse-flow type regenerative pyrolysis reactor system.
U.S. Pat. No. 7,491,250 to Hershkowitz et al. discloses production of synthesis gas through a cyclic, packed-bed operation which includes reforming by preheating a first zone, introducing a hydrocarbon-containing feed with steam through the first zone inlet, and reforming over a catalyst in the first zone to form synthesis gas which is passed to a second zone where it is cooled.
U.S. Patent Application Publication No. 2007/0144940 to Hershkowitz et al. and U.S. Patent Application Publication No. 2008/0142409 to Sankaranarayanan et al. teach a regenerative bed reverse flow reactor wherein the location of the exothermic reaction is controlled. The regenerative reactor bed is regenerated by supplying a first reactant through a first channel to a first regenerative bed and a second reactant through a second channel in the first regenerative bed, combining first and second reactants in a gas mixer, and reacting to produce a heated reaction product which is passed through a second regenerative bed to transfer heat thereto.
U.S. Patent Application Publication No. 2009/008292 to Keusenkothen et al. teaches pyrolyzing hydrocarbons containing non-volatiles in a regenerative pyrolysis reactor system. Feedstock is heated to provide a vapor phase which is fed to the pyrolysis reactor system and converted to form a pyrolysis product.
U.S. Patent Application Publication No. 2008/0314550 to Greco teaches a regenerative heat exchanger that uses inlet and outlet poppet valves which are operated in tandem and located outside the head space of the heat exchanger.
Accordingly, it is desirable to provide a reverse-flow reactor system that minimizes dead volumes between its valves and reactor beds, while providing extended valve lifetimes to millions of cycles, in rugged, high-temperature conditions at the reactor inlet and outlet. Further, there is a need for an enhanced method and apparatus to implement an industrial-scale, high-GHSV RFR's, which has valves that enhance the cycle time of RFRs and manage the purging of fluid between cycles. The present techniques provide a method and apparatus that overcome one or more of the deficiencies discussed above.